1. Field of Invention
The present invention generally relates to semiconductor devices and more particularly to highly efficient photovoltaic (PV) cells with selective emitters.
2. Description of Related Art
A crystalline silicon photovoltaic (PV) cell typically has a front side surface operable to receive light and a back side surface opposite the front side surface. The front side surface is part of an emitter of the PV cell and has a plurality of electrical contacts formed therein. The back side surface has at least one electrical contact. The electrical contacts on the front and back sides are used to connect the PV cell to an external electrical circuit.
The front side contacts are typically formed as a plurality of parallel spaced apart “fingers” that extend across the entire front side surface. The fingers are formed by screen printing a metallic paste onto the front side surface in a desired pattern. The metallic paste is diffused into the front side surface such that only a small portion of the paste is left on the front side surface and this small portion is seen as the fingers or above described lines. Additional paste may be provided to create bus bars that extend at right angles to the fingers, to collect electric current from the fingers. The bus bars are typically wider than the fingers to enable them to carry the current collected from the fingers.
The electrical contacts and bus bars are opaque and shade the emitter from light, which reduces the effective emitter area available for light gathering. As a result, the area that is occupied by the screen printed fingers and bus bars on the front side of the substrate is known as the shading area because the opaque paste that forms the fingers and bus bars prevents solar radiation from reaching the emitter in this area. The shading area reduces the current producing capacity of the device. Modern solar cell substrate shading areas occupy 6% to 10% of the available active surface area.
Although silicon crystalline cells are produced in large volumes, there exists a need to increase their efficiency and decrease their production cost in order to make photovoltaic energy cost competitive. Optimization of front side metallization is one way to decrease the shading area that is occupied by metallic contacts. A decrease in shading area increases the electric current and voltage of a PV cell since it increases the surface area of the substrate that is reached by solar radiation and it also reduces the diffusion of the contact paste into the front surface of the substrate, the diffusion having a detrimental effect on charge recombination. Charge recombination on front and back sides of PV cells may be substantially reduced by passivation with thin layer dielectric materials, such as for example SiO2, SiNx, SiC by employing industrially available technologies and equipment (S. W. Glunz et. al., “Comparison of different dielectric passivation layers for application in industrially feasible high-efficiency crystalline solar cells” presented at the 20th European Solar Conference and Exhibition, 6-10 Jun., 2005, Barcelona).
Conventional screen printing technology imposes limitations on solar cell efficiency improvements due to a restriction on emitter thickness. When the emitter thickness is less than a diffusion depth of the metallic paste during the screen printed fingers firing process, electrical shunting through the p/n junction occurs. Therefore modern screen printing technology allows the production of solar cells with emitter sheet resistance of typically no more than 65 Ohm/sq. This corresponds to an emitter thickness of greater than 0.2 micrometers. At the same time it is known that an emitter with a sheet resistance of greater than 100 Ohm/sq sheet resistivity and thickness of less than 0.2 micrometers provides a substantial gain in cell efficiency mainly due to lower optical losses in the blue spectral region. An emitter with these properties is known as a shallow emitter. Thus, in order to increase the conversion efficiency of solar cells that employ a conventional screen printed metallization, emitter design parameters may be optimized such that under a screen printed finger, an emitter thickness is sufficiently high while in light-illuminated areas, the emitter thickness is substantially thinner. An emitter with these differing thicknesses is known as a selective emitter. In a selective emitter, sufficient emitter thickness and high dopant concentration in areas under current collecting fingers and bus bars ensures low resistance electrical contacts between the semiconductor substrate and the fingers and bus bars without shunting the p/n junction. Although the use of a selective emitter has proved to be effective in improving PV cell efficiency, implementation of a selective emitter in practice, is quite complicated.
Another approach to improving solar cell performance comprises etching back a dead zone of the emitter, leaving only a zone of decreasing dopant concentration in the emitter. A dead zone, or zone of relatively constant dopant concentration is formed in semiconductor material when a dopant is diffused into the material. A zone of decreasing dopant concentration is formed immediately adjacent the dead zone. The dead zone has a relatively high dopant concentration. In this zone, recombination of electric charges occur quite readily, which is undesirable. Therefore it is common in the art to try to remove this zone using conventional etching methods, to leave only the zone of decreasing dopant concentration. Conventional etch back methods are based on wet etching or plasma etching processes that involve high temperatures, which requires expensive equipment and special procedures and not compatible with multi crystalline silicon semiconductor material. Moreover, these methods can result in partial thinning of the zone of decreasing concentration. As a result, the thickness of the emitter cannot be accurately controlled and thus manufacturing tolerances are difficult to achieve in production.
U.S. Pat. No. 5,871,591 entitled “Silicon solar cells made by a self-aligned, selective-emitter, plasma-etchback process” to Ruby et al. describes a process for forming and passivating a selective emitter. The process uses a plasma etch of a heavily doped emitter to improve its performance. Screen printed metallic patterns, also referred to as grids of the solar cell, are used to mask a plasma etch such that only regions of the emitter between the grids are etched, while regions beneath the grids remain heavily doped to provide low contact resistance between the substrate and the screen printed metallic grids. The process is potentially a low-cost process because it does not require precision alignment of heavily doped regions with screen printed patterns. After the emitter is etched, silicon nitride is deposited by plasma-enhanced chemical vapor deposition, to provide creating an antireflection coating. The solar cell is then annealed in a forming gas. The proposed plasma etchback method provides for a substantial decrease in dopant concentration on the emitter surface which improves an emitter doping profile and provides a corresponding improvement in solar cell efficiency due to reduced surface charge recombination. While this method allows fabrication of a selective emitter and an increase in solar cell efficiency, it has the disadvantage that it is unable to provide sufficient control over the final thickness of the emitter after etchback processing. This disadvantage results in poor reproducibility of properties of the produced PV cells.
U.S. Pat. No. 6,091,021 entitled “Silicon solar cells made by a self-aligned, selective-emitter, plasma-etchback process” to Ruby et al. describes PV cells and a method for making the PV cells wherein metallized grids of the PV cells are used to mask portions of PV cell emitter regions to allow selective etching of the regions. Self-aligned selective etching allows for an enhanced blue response as compared to PV cells with uniform heavy doping of the emitter, while preserving heavier doping in the regions beneath the gridlines, as is needed for low contact resistance. The method may replace difficult alignment methodologies used to obtain selectively etched emitters, and may be easily integrated with existing plasma processing methods and techniques.
The method provides for a substantial decrease in a doping concentration on the emitter surface which improves the emitter doping profile and provides a corresponding improvement in solar cell efficiency due to reduced surface charge recombination. However, again, the proposed method is unable to provide sufficient control over the final thickness of the emitter after the etchback processing resulting in poor reproducibility of properties of the produced PV cells.
U.S. Pat. Nos. 6,552,414 and 6,825,104 both entitled “Semiconductor device with selectively diffused regions” to Horzel et al. describe a PV cell having two selectively diffused regions with different doping levels. A first screen printing process is used to deposit a solid based dopant source onto a substrate. Diffusion of dopant atoms from the dopant source into the front side of the solar cells is arranged in a specially provided atmosphere to produce two areas with different dopant concentrations: a high dopant concentration area under the dopant source, and a low dopant concentration area on the rest of the solar cell's front side. A second screen printing process deposits a metallization pattern that is precisely aligned to ensure that screen printed fingers and bus bars are in electrical contact with the high dopant concentration areas of the emitter. However, with these methods, it is very difficult to ensure sufficient reproducibility of the properties of the emitter, especially the thickness of the selective shallow emitter region.